The main points of the author's thoughts in The Future of the Universe

This article is Hawking's speech at Cambridge University in June 199 1.

This speech, from ancient wizards' predictions to modern religious predictions, and then to the discussion of the future of the universe, mainly discusses two kinds of fate of the future of the universe: one is to continue to expand, and the other is to shrink or even collapse into a point. Expansion or contraction depends on the average density of the universe. Using the theory of astrophysics to make a scientific diagnosis of the future of the universe is the difference between scientists and wizards.

The theme of this speech is the future of the universe, or rather, what scientists think the future will look like. It is of course very difficult to predict the future. I once had an idea to write a book called Yesterday's Tomorrow: The History of the Future. This will be a history of prediction about the future, and almost all the predictions are wrong. But despite these failures, scientists still think they can predict the future.

Early on, it was the duty of a prophet or a witch to predict the future. These women are usually hypnotized by poisonous gas or gas escaping from the cracks in the volcano. The priests around translated their spells. The real skill is to explain. The famous Dravidian in ancient Greece was notorious for being ambiguous. When these Spartans asked the Persians what would happen if they attacked Greece, the wizard replied: Either Sparta was destroyed or its king was killed. I think these priests thought that if all this didn't happen in the end, Sparta would be so grateful to Apollo that he ignored the fact that his wizard made a wrong prediction. In fact, the king was killed in an operation to save Sparta, and finally defeated the Persians by defending the Tedipyra Pass.

On another occasion, Dick Rossius, the king of Lidya, the richest man in the world, once asked what would happen if he invaded Persia. The answer is: a great kingdom will collapse. Crocy uz thought that this refers to the Persian Empire, but he didn't know that it was his own kingdom that was going to fall, and his own fate was to be burned alive on the pyre.

Modern doomsday prophets don't set a date for the end of the world to avoid embarrassment. These dates caused the stock market to plummet. Although it puzzles me, why does the end of the world make people willing to exchange stocks for money, assuming you can't take anything with you at the end of the world?

So far, all the set dates of the end of the world have passed silently. But these prophets often make excuses for their obvious failures. For example, william miller, the founder of the Seventh Day Return, predicted that the second coming of Jesus would take place between1March 2, 8431and1March 2, 8441. After this did not happen, the date was changed to1844101October 22nd. When this date passed and nothing happened, a new explanation was put forward. It is said that 1844 is the beginning of the second regression, but first we should count the names of the rescued people. Judgment day will only come to those who are not on the list when the list is counted. Fortunately, counting names seems to take a long time.

Of course, scientific predictions are not necessarily more reliable than those of wizards or prophets. People only need to consider the weather forecast. But in some cases, we think we can make reliable predictions. The future of the universe on a very large scale is an example.

In the past 300 years, we have discovered the scientific laws governing objects under all normal conditions. We still don't know the exact laws governing objects under extreme conditions. Those laws are important for understanding how the universe began, but they will not affect the future evolution of the universe unless the universe collapses to a high-density state. In fact, we have to spend a lot of money to build a giant particle accelerator to test these high-energy laws, which shows how insignificant their impact on the present universe is.

Even if we know the laws of the universe, we still can't use them to predict the distant future. This is because the solution of the physical equation will present a property called chaos. This shows that the equation may be unstable: if the system changes very slightly at a certain moment, the future behavior of the system will soon become completely different. For example, if you change the way you spin the roulette wheel a little, you will change the numbers, which you can't actually predict, otherwise physicists will make a fortune in the casino.

In unstable or chaotic systems, there is generally a time scale, in which the small changes in the initial state will be doubled. In the case of the earth's atmosphere, this time scale is about five days, which is about the time for air to circle the earth once. People can make a fairly accurate weather forecast within five days, but to make a longer-term weather forecast requires both an accurate understanding of the current atmospheric conditions and insurmountable complex calculations. Apart from the quarterly average, we can't make a specific weather forecast six months later.

We also know the basic laws governing chemistry and biology, so in principle, we should be able to determine how the brain works. But the equations that control the brain almost certainly have chaotic behavior, and very small changes in the initial state will lead to very different results. In this way, although we know the equations that constrain human behavior, we can't predict them in practice. Science cannot predict the future of human society, or even whether it has a future. The danger is that our ability to destroy or destroy the environment is growing much faster than the wisdom of using this ability.

The rest of the universe doesn't care about anything that happens on earth. It seems that the movement of planets around the sun will eventually become chaotic, although its time scale is very long. This shows that with the passage of time, the error of any prediction will become bigger and bigger. After a while, it is impossible to predict the details of the movement. We can be quite sure that the earth will not collide with Venus for a long time. However, we are not sure whether small disturbances in orbit will accumulate and lead to this collision after more than one billion years. The motion of the sun and other stars around the Milky Way, as well as the motion of the Milky Way around its galaxy cluster, is also chaotic. We observe that other galaxies are moving away from us. The farther they are from us, the faster they leave. This means that the universe around us is expanding: the distance between different galaxies increases with time.

The background of microwave radiation from outer space we observed gives evidence that this expansion is stable, not chaotic. You can observe this radiation by tuning the TV to an empty channel. A small number of spots you see on the screen are caused by microwaves outside the solar system. This is the same kind of radiation emitted by microwave ovens, but it is much weaker. It can only heat food to an absolute temperature of 2.7 degrees, so it can't be used to warm your takeaway pizza. This radiation is thought to be a remnant of the early thermal universe. But the most impressive thing is that the amount of radiation in any direction is similar. The cosmic background detection satellite has measured this radiation very accurately. According to these observations, the sky map can show different radiation temperatures. These temperatures are different in different directions, but the difference is very small, only one in 100 thousand. Because the universe is not completely smooth, there are local irregularities such as stars, galaxies and clusters of galaxies, so microwaves in different directions are bound to be somewhat different. But in order to match the local randomness we observed, the change of microwave background can't be smaller. The microwave background can be equal to 65438+99999 in all directions.

In ancient times, people thought that the earth was the center of the universe. The fact that the background is the same in any direction is not surprising to them. However, since the Copernican era, we have degenerated into a planet, revolving around a very ordinary star, which in turn revolves around the outer edge of a typical galaxy in the 1000 billion galaxies we can see. We are too modest now to claim any special position in the universe. So we must assume that the background is the same in any direction around any other galaxy. This is only possible if the average density and expansion rate of the universe are the same everywhere. Any change in the average density or expansion rate of a large area will make the microwave background in different directions different. This shows that the behavior of the universe is simple on a very large scale, not chaotic. So we can predict the distant future of the universe.

Because the expansion of the universe is so uniform, people can describe it with a single number, that is, the distance between two galaxies. Now this distance is increasing, but it is expected that the gravitational attraction between different galaxies is reducing this expansion rate. If the density of the universe is greater than a certain critical value, gravity will eventually stop expanding and the universe will begin to shrink again. The universe will collapse into a big squeeze, which is very similar to the big bang at the beginning of the universe. Big extrusion is something called singularity, which is a state with infinite density, and the laws of physics fail in this state. This shows that even if something happens after the big squeeze, their fate is unpredictable. But if there is no causal connection between events, there is no reasonable way to say that one event happens after another. Maybe people can say that our universe ended in a big squeeze, and everything that happened "after" is part of another separated universe. Kind of like reincarnation. If someone claims that a newborn baby is equivalent to a deceased person, what is the point of this statement if the baby has not inherited any characteristics or memories from his previous life? People can tell him that this is a completely different individual.

If the density of the universe is less than the critical value. It will not collapse, but will continue to expand forever. Its density will become so low after a period of time that gravity has no significant effect on slowing down the expansion. Galaxies will continue to move away from each other at a constant speed.

In this way, the key question of the future of the universe is: what is the average density? If it is less than the critical value, the universe will expand forever. But if it is greater than the critical value, the universe will collapse and time itself will end in a big squeeze. However, I have an advantage over other doomsday prophets. Even if the universe is about to collapse, I can confidently predict that it will not stop expanding for at least10 billion years. I didn't expect that I would stay in the world at that time and be proved wrong.

We can estimate the average density of the universe from observations. If we calculate the visible stars and add up their masses, we get less than 1% of the critical value. Even if we add the mass of gas clouds observed in the universe, it still only adds the total to 1% of the critical value. However, we know that the universe should still contain so-called dark matter, that is, things that we can't directly observe. One evidence of dark matter comes from spiral galaxies. There are huge cake-like stars and gas polymers. We observed them rotating around the center. But if they only contain the stars and gases we observe, the rotation rate is high enough to throw them away. There must be some invisible form of matter, and its gravity is enough to firmly grasp these rotating galaxies.

Another evidence of dark matter comes from galaxy clusters. We observe that galaxies are unevenly distributed in the whole space, and they gather together, ranging from a few galaxies to hundreds of galaxies. Suppose these galaxies attract each other and form clusters of galaxies. However, we can measure the speed of individual galaxies in these clusters. We find that its speed is so fast that if there is no gravity to pull them together, these clusters of galaxies will disperse. The required mass is much greater than the total mass of all galaxies. This is estimated in this case, that is, we think that galaxies have the mass needed to keep themselves together when they rotate. Therefore, there must be extra dark matter outside the galaxies we observed in the cluster.

People can make a fairly reliable estimate of the amount of dark matter in galaxies and clusters of galaxies for which we have definite evidence. But this estimate is still only about 10% of the critical mass that makes the universe collapse again. In this way, if we only rely on observational evidence, we can predict that the universe will continue to expand indefinitely. In about 5 billion years, the sun will run out of its nuclear fuel. It will expand into a so-called red giant until it engulfs the earth and other closer planets. It will eventually stabilize into a white dwarf with a scale of only a few thousand miles. I'm predicting the end of the world, but not yet. This forecast did not make the stock market too depressed. There are one or two more pressing problems ahead. Anyway, assuming that we haven't destroyed ourselves at the moment when the sun explodes, we should have mastered the technology of interstellar travel.

After about10 billion years, most stars in the universe have run out of fuel. Stars with the mass of the sun are either white dwarfs or neutron stars, which are smaller and denser than white dwarfs. More massive stars will become black holes. Black holes are smaller, and the gravitational field is too strong for light to escape. However, these residues continue to revolve around the center of the Milky Way every 1 100 million years. The collision of these remnants will cause some to be thrown out of the galaxy. The rest will gradually stabilize in a closer orbit near the center, and final gathering together will form a huge black hole at the center of the galaxy. No matter what dark matter is in galaxies or clusters of galaxies, it can be expected that they will also fall into these very huge black holes.

Therefore, it can be assumed that most objects in a galaxy or cluster of galaxies eventually exist in black holes. However, a few years ago, I found that black holes are not as black as they are painted. According to the uncertainty principle of quantum mechanics, it is impossible for a particle to have a well-defined position and a well-defined velocity at the same time. The more precise the position of a particle is, the less precise its velocity is, and vice versa. If there is a particle in a black hole, its position in the black hole is well defined, which means that its velocity cannot be accurately defined. So the speed of particles may exceed the speed of light, making it escape from the black hole, and the speed of particles and radiation leaking from the black hole is so slow. The giant black hole at the center of the galaxy may be millions of miles in size. In this way, the position of particles in it has great uncertainty. So the uncertainty of particle velocity is very small, which shows that it takes a very long time for a particle to escape from a black hole. But it will escape eventually. The giant black hole in the center of the galaxy may take 1090 years to evaporate and disappear completely, that is, "1" is followed by 90 zeros. This is much longer than the age of the universe now It is 10 10 year, which means "1" followed by 10 zeros. If the universe is to expand forever, black holes still have enough time to evaporate.

The future of the expanding universe is rather boring. But whether the universe will expand forever is far from certain. We only have conclusive evidence that the density required for the collapse of the universe is one tenth. However, there may be other kinds of dark matter that have not been detected by us, which will make the average density of the universe reach or exceed the critical value. These extra dark matter must be outside the galaxy or cluster of galaxies. Otherwise, we should be aware of its influence on the rotation of galaxies or the motion of galaxies in galaxy clusters.

Why do you think there may be enough dark matter to finally collapse the universe? Why can't we believe that we have conclusive evidence? The reason is that even though the universe is only one tenth of the critical density now. You need to be very careful when choosing the initial density and expansion rate. If the density of the universe increases by one trillion one second after the Big Bang, the universe will collapse ten years later. On the other hand, if the density of the universe is reduced by the same amount at that time, the universe will become basically empty in about ten years.

Why is the initial density of the universe chosen so carefully? There may be some reasons why the universe must have a critical density. It seems that there may be two explanations. One is the so-called anthropic principle, which can be restated as follows: the universe is like this, because otherwise, we would not observe it here. The idea is that there may be many different universes with different densities. Only those substances that are very close to the critical density can survive long enough and contain enough substances to form stars and planets. Only in those universes can intelligent creatures ask the question: Why is the density so close to the critical density? If this is the explanation of the current density of the universe, there is no reason to believe that the universe contains more matter than we have detected. One tenth of the critical density is enough to form galaxies and stars.

However, many people don't like the anthropic principle because it seems to rely too much on our own existence. Therefore, some people seek another possible explanation to explain why the density should be so close to the critical value. This exploration led to the expansion theory of the early universe. The idea is that the size of the universe has doubled, just as in countries suffering from extreme inflation, prices will double every few months. But the explosion of the universe is much faster and more extreme: an increase of at least 1000 billion times will make the universe so close to the accurate critical density that it is still very close to the critical density. Thus, if the expansion theory is correct, the universe should contain enough dark matter to make the density reach a critical value. This means that the universe may eventually collapse, but this time will not be much longer than the fifteen billion that has expanded so far.

To sum up, scientists believe that the universe is governed by clearly defined laws, which in principle allow people to predict the future. But the movement given by law is usually chaotic. This means that a small change in the initial state will lead to a rapid increase in subsequent behavior. In this way, people can only make accurate predictions for a relatively short time in the future. However, the large-scale behavior of the universe seems to be simple rather than chaotic. Therefore, people can predict whether the universe will expand forever or eventually collapse. It depends on the existing density of the universe. In fact, the current density seems to be very close to the critical density that distinguishes collapse from infinite expansion. If the inflation theory is correct, the universe is actually on the cutting edge. So I just inherited the good tradition of those wizards or prophets, and both sides made a bet to show their affirmation.